Reallocation will be the name of the game in months ahead as federal agencies figure out how to live with President Carter's tough new hiring controls.

Under the economy-in-government plan, Carter has told departments they can fill only half of their job vacancies in the future. The order, being enforced by the Office of Management and Budget, wiped out all existing job vacancies-estimated at between 20,000 and 30,000-in government last week.

There will be exceptions to the one out of two replacement rule, but OMB brass say that they will be few and far between. For the most part, according to the game plan, agencies and offices are going to be limited to hiring only one replacement for every two persons who leave. Normally about 14,000 U.S. workers leave government, for a variety of reasons, each month.

Agencies will have authority, in most cases, to pick which jobs they want filled so long as they abide by the overall 50 percent replacement policy. That is where "reallocation" comes in. Some offices, for examples, will continue hiring as per usual. Others will be frozen solid and not allowed to replace anyone who leaves.

Secretary's of several major departments have already put out the word that their immediate offices must have replacements as necessary. That means the 50-50 replacement rule will be jiggled by top personnel officials so that headquarters operations, and programs dear to the Secretary's (and congressional committee chairmen's) hearts will keep rolling along, fully staffed. To make up for it, some offices will be told they can fill only one of every four or five vacancies as they occur.

No indication yet how long the partial hiring freeze will remain in effect. Officials say that in the first few months it should trim about 7,000 jobs each month. After that, they expect the attrition rate to decline, with resulting smaller jobs savings.

The key to the effectiveness of the partial freeze will be how many ex-emptions are granted. So far there have been none. If some agencies-IRS and Federal Avistion Administration are major examples-are allowed to hire as usual the pressure on OMB to grant additional exemptions will be great.

FAA and IRS traditionally have been sparred the full effects of past makes money. The other keeps air-planes from running into each other. Most agencies have a tougher time justifying exemptions from freezes-not having important $$$ or life-saying missions. But portions of HEW, VA and Defense, three of the biggest operations, are planning to ask for major exemptions.